Composite materials are used routinely in the aerospace, automotive and sporting goods industries, to name just a few examples. Structural composite articles are lighter, offer better corrosion resistance, and display improved resistance to damage from cyclic loading (fatigue) compared to corresponding articles that are formed from machined metals. For instance, replacing an aluminum structure with graphite/epoxy composite may achieve a weight reduction of greater than 20%.
Unfortunately, relatively high costs are associated with the manufacture of composite articles. A typical process begins with the preparation of a preimpregnation, or prepreg, which is a ready-to-mold sheet of fiber reinforcement that has been preimpregnated with a resin system. The resin component of a prepreg is usually thermosetting, but thermoplastic prepregs are available as well. For thermosetting prepregs, the resin is partially cured to a B-stage condition and supplied to the fabricator, who lays up the finished shape using the required layers of prepreg. The fabricator then completes the cure with heat and pressure.
The majority of thermoset prepegs require refrigeration to a temperature as low as −20° C. (approximately 0° F.) to preserve their physical and chemical characteristics. As such, prepegs are normally transported in refrigerated trucks or packed in dry ice for air transport. Further, it is necessary to store the prepegs at low temperature even after delivery to the fabricator. When part fabrication begins, the prepreg is removed from cold storage and warmed to room temperature. Remaining quantities of prepreg must be returned to cold storage. Out-time, which refers to how long a prepreg can remain out of cold storage and still retain its properties without degrading, must be documented for critical applications. If out-time nears or exceeds the manufacturer's specified shelf life, the material must be retested and recertified or disposed of, both of which are costly options. The need to store and transport thermosetting prepreg under conditions of low temperature, as well as the cost of disposing of degraded prepreg, limits the use of fiber reinforced composite articles to a relatively few applications with specialized requirements.
Although prepregs that are based on thermoplastic materials do not require refrigeration to a temperature as low as −20° C. during transportation and storage, they are nevertheless susceptible to degradation under certain conditions. In particular, the heat history of a thermoplastic prepreg is an important consideration. It is therefore a disadvantage of prior art systems that a thermoplastic prepreg is produced at elevated temperature and then subsequently cooled to about room temperature for storage and shipping. After the prepreg is transported to a production facility, it must be heated up again to a suitable forming temperature. Since a thermoplastic prepreg degrades during each heating and cooling cycle it is subjected to, the properties of the final product that is formed from such a prepreg may be compromised. Another disadvantage that is associated with cooling and reheating the thermoplastic prepreg relates to energy wastage. In particular, the heat that is provided to form the thermoplastic prepreg is wasted because the prepreg is allowed to cool down for storage or transportation, and then it must be reheated during the operation to form the composite article.
Other known processes for the manufacture of composite articles include Resin Transfer Molding and Resin Infusion Molding. In both of these processes the sheets of fiber reinforcement are arranged in a mold, which is then closed prior to the sheets being wetted with resin. The resin is subsequently injected into the mold during forming of the final product. Unfortunately, processes in which the resin material is injected directly into the mold tend to be slow and/or uniform wetting of the sheets of fiber reinforcement may not occur. As a result, these methods tend to be used to produce parts that do not require high quality or defect free surfaces.
It would be beneficial to provide a system and method for forming composite articles that overcome at least some of the above-mentioned limitations of the prior art.